I was wondering if anyone knew how to setup dosbox to have the classic midi sound for descent. For some reason i think it sounds better than the wavetable version of the music.

Type Setup, select "Change Midi Device", and then select OPL2 (or OPL3)

Also, if you have a fairly beefy PC, follow my tips in this thread to get DOSBOX working with Timidity++, which is generally better than the worthless midi included with most computers (disregard if you have a Creative card, they actually have half-way decent(or was that descent:) ) midi). Also, if you do some searching, you can find a soundfont, a collection of samples that the midi synth will use, that is the original set of samples for descent. It should work with either Timidity or a Creative card.

In the MIDI/Music card selection, select "General Midi" or "MPU-401" if you can't find General Midi, design port as "0x330" (or 330) and you should hear the music as your sound card always played it back in the day.
This works for all dos games.

You want the old FM synthesis mode, I think, not the MIDI mode. And the Guest has it right - you want to change the music source to OPL3 (or OPL2 if that doesn't work). You can find the 'Setup' program in the Descent folder where you installed the game.
Incidentally, I totally agree - despite the fact that the wavetable synthesis does sound better in most instances, the composers really did a great job with the OPL version of the songs.

If you just wanted to turn off the redbook cd music and go back to the classic MIDI score, a very simple way to do it is go to the descent 2 main menu, then go options>sound effects and music>, and then deselect the box labled "CD MUSIC (REDBOOK) ENABLED.